Field of the Invention
The invention relates to a process for the non-destructive determination of the stressing condition of ferromagnetic securing elements, in which process an alternating magnetic field is applied to said elements and the magneto-mechanical acoustic emission thereof is detected and evaluated.
Determining the stressing condition which occurs in a securing element at its site of use is a topical problem of vast significance. Its significance resides above all in the aspect of safety. However, the economic aspect of this problem is not to be underestimated, either. In this context, it may also be pointed out that all over the world, product liability has been playing an ever-increasing part.
What is especially significant is the determination of the stressing conditions prevailing in screwed connections. Whenever parts are to be detachably connected, this is as a rule done by means of screwed connections. In a motorvehicle, for example, about two to three thousand screwed connections can be found. Of these, about three hundred are crucial for safety. For this reason, these screws or bolts have to be tightened particularly carefully, i.e. nether too loosely nor too strongly (cf. "Industrieanzeiger" 13/1988, pp. 24 through 27).
When a screw or bolt is tightened, the ultimately decisive factor is the tension force. It is the tension force alone which decides on how tightly the parts are to be detachably connected. At present, the man skilled in the art does not disposed of any direct methods for measuring this force; he can merely conclude from indices or auxiliary magnitudes, namely the torque or the pivoting angle or a combination of the two. Furthermore, the yield point may also be considered to be an auxiliary magnitude in a certain sense.
When a screw or bolt is tightened under torque control, it must be taken into account that sixty to ninety per cent of the torque are required as friction under the screw or bolt head and in the thread. It is therefore the remaining ten to forty per cent of the torque alone which effect a strain on the screw or bolt. This strain on the screw or bolt alone causes the desired tension force. Under different friction conditions and with constant tightening moment, the tension force can vary up to a hundred per cent in the extreme case. For verifying the tension force of a screwed connection, detecting the torque thus remains merely a makeshift measuring solution. Deviations from the required tension force in screwed connections can be reduced by the use of a tightening process which is controlled by both the torque and the pivoting angle. In this process, however, the expenditure for electronic controlling and measuring equipment is far higher than with torque-controlled tightening. In a tightening process which is controlled by the torque/pivoting angle, the screw or bolt is turned in until a plastic deformation occurs, from which point onwards the screw or bolt is tightened by a predetermined angle. Since the strain of the screw or bolt can be determined from the pivoting angle and the pitch of the thread, the tension force of the screw or bolt may be calculated, taking into account the elastic modulus of the screw/bolt material. Irrespective of the friction conditions, this yield point-controlled tightening process provides the best indirect indications about the pre-stressing force of a screwed connection.
As a rule, it would be ideal for determining the prestressing force if the parameters stress and strain could be measured directly during the tightening of the screw or bolt. In production, however, when conventional tightening processes were used, it has so far been impossible to measure these parameters ("Industrieanzeiger" 16/1988, pp 21 through 24).
German patent application DE-AS 25 19 430 already discloses a measuring process for determining the axial force of screw bolts in which the screw bolts are excited to ultra-sonic resonance vibrations by means of transversal and longitudinal waves. The natural frequencies are thus dependent on the longitudinal modulus and the transversal modulus, resp. of the screw bolts. The axial forces to be measured are derived from shifts of the natural frequencies as a result of the strain of the sample body.
Moreover, German patent DE 33 33 285 A1 teaches means for detecting forces, stresses and accelerations in machines, devices and the like, in which a measuring value generator is built into a structural element under stress. The measuring value generator contains a low-Ohm resistance structural element. In screwed connections, the measuring value generator is built into the bolt or the washer.
Further, German patent DE 33 45 760 C2 discloses measuring means comprising a measuring spring made of ferromagnetic material, in which the deformation behaviour of an elastic spring of ferromagnetic material is examined by means of a strain measuring strip and a magnetic field which passes through the spring at the site of the strain measuring strip.
German patent DE 27 14 334 A1 teaches means for monitoring the tension force of a screwed connection, in which a tightly fitting measuring wire is wound around the periphery of the nut of a screwed connection for measuring the stress-related radial expansion. One end of the measuring wire is secured to the nut, the other end remaining tangentially movable on its periphery. After the prestressing force has been exerted on the nut, the latter expands radially, resulting in the measuring wire being displaced. This displacement is detected longitudinally and assigned to the prestressing force.
Furthermore, German utility model DE-GM 6608345 discloses a detecting element in the form of a washer which transforms the applied prestressing force into an electrical signal.
For detecting stresses in materials having ferromagnetic properties, magnetic measuring processes may be used. These involve, for example, recording the Barkhausen effect, measuring the permeability or multiple detecting of magnetic parameters. The physical causes of the stress susceptibility of ferromagnetic materials are based on magnetoelastic effects which, after material deformation or interior rearrangement processes, result in changes in the energy contributions determining the domain structure. These changes then influence the magnetic parameters.
German patent DE 34 04 232 relates to a process for examining material properties and material states of ferromagnetic materials on the basis of the magneto-mechanical acoustic emission of the ferromagnetic material in a magnetic field. In this process, the magneto-elastically induced emission of a standardized ferromagnetic sample and of the ferromagnetic material to be examined is recorded at a continuously varied frequency of the magnetic field and at a constant amplitude of the magnetising field strength of the alternating field, after which the acoustic emissions of the standardized ferromagnetic sample and the emissions emanating from the ferromagnetic material under test are then compared. In this process, a modifiable portion of a direct-current field is also intended to be superimposed on the alternating magnetic field. This modifiable portion of the direct-current field is used to adjust a certain point of the hysteresis curve for premagnetizing the sample or for demagnetising a previously magnetized sample.
It may further be gathered from a report of a meeting (W. Stengel: "Magnetoelastische Resonanzspektroskopie". Deutsche Gesellschaft fur zerstorungsfreie Prufung e.V. (German Society for non-destructive Testing), Berichtsband (Volume) 14, 1988, pp 638 through 645) that a sample which is subjected to a tensile force is subsequently found to have a characteristic magnetostrictive vibration behaviour in every area of the corresponding stress-strain diagram. Plastic deformation of the sample can be seen from a frequency shift of related resonance amplitudes. In this case, the magnetostrictive vibration was induced exclusively along the longitudinal measuring area of the tensile sample by means of a cylindrical coil surrounding the longitudinal measuring area. The acoustic emission behaviour thus can only be related to the tensile stress condition prevailing in this area.